When they "clean" highway bridges

[longhorn]

I have noticed when a bridge is about to be torn down or expanded they "clean" the bridge of grime or black soot. Not sure its sand blasting or chemical. Why do they "clean" the bridge before expansion or demolition?

[inkyatari]

Topical, as they're about to clean the IL-47 bridge over the Illinois river.

[Chris]

This may have to do with the removal of a paint additive used widely until the 1980s: Chromium VI, also known as hexavalent chromium. It was applied widely as a corrosive protection but was later found to be carcinogenic

See also: https://www.osha.gov/Publications/OSHA_FS-3649_Bridge_Painting.pdf